Wavelength-division multiplexing (WDM) networks operate by multiplexing different wavelength optical carrier signals onto optical fibers using different wavelengths. A WDM network employs WDM equipment such as Optical Cross-Connects (OXCs), Optical Add/Drop Multiplexers (OADMs), Reconfigurable OADMs (ROADMs), optical amplifiers, in order to establish end-to-end lightpaths (or “lambdas”) between optical terminal equipment to form a virtual topology over the physical fiber topology. The WDM network in this way operates as an optical transport layer and may be dynamically reconfigured to provide lightpaths (also known as “lambdas” or “wavelengths”) responsive to the traffic needs of client layers. The three distinct network layers—packet, circuit switching, and optical transport—have evolved towards a model where only two layers remain in the majority of networks: IP packets (routers) being transported over WDM (optical transport). Circuit switching (e.g., SONET/SDH) has either been removed entirely as packet traffic has become the dominant traffic type, or its function has been subsumed into optical transport network (OTN) switching embedded into optical transport systems.
Some optical networks allocate a separate wavelength to carry management and control information. Per the International Telecommunication Union (ITU-T) G.692 Recommendation, this Optical Supervisory Channel (OSC) is defined as a channel that is accessed at each optical line amplifier site that is used for maintenance purposes including (but not limited to) remote site alarm reporting, communication necessary for fault location, and orderwire. The Optical Supervisory Channel is not used to carry payload traffic.
The OSC is typically implemented as an additional wavelength outside the optical amplifier band in order to separate network management and control information from user data. In other words, the OSC may be an in-fiber, out-of-band supervisory channel. Management messages for the photonic layers (e.g., Optical Multiplex Section, Optical Transmission Section, and Optical Channel, as defined by ITU-T G.709) may all be sent on a single, common OSC. The OSC carries information about the WDM optical signal as well as remote conditions at the optical terminal or amplifier site. The OSC is also often used for remote software upgrades and network management information. The OSC generally terminates at each optical network element, including intermediate amplifier sites, where the optical network element adds local information for the optical network element before retransmitting.